How do I use Windseeker perk Born Again?

I saw a thread on this forum that says Red Wedding isn’t supposed to trigger Born Again because Red Wedding happens after start-of-battle effects. Barbearian also says it happens after start-of-battle effects. I used a Chaos Salamander to cast Slaughter and kill 5 of my creatures during start-of-battle, and that didn’t trigger it.

What triggers born again? How can it be used?